a. Ohm's law states that the current flowing in a DC circuit is directly proportional
to the EMF and inversely proportional to the resistance of the circuit. Ohm's law may be
expressed by anyone of three equations. In common usage, I is current expressed in
amperes, E is EMF expressed in volts, and R is resistance expressed in ohms.
I = _E_
R = _E_
Thus, to find a resistance when E and I are known, use R = E/I (volts divided by
amperes) to find the voltage. When I and R are known, use E = I X R. If any two of
these values are known, it is possible to find the value of the unknown by using one of
these formulas. A simple aid to memorizing Ohm's law formulas is to cover the
unknown factor in the formula __E__ and see what mathematical manipulations
are required to find it. For example, if it is desired to find the resistance (R) when the
applied voltage (E) is 12 volts and the current (I) is 3 amperes--cover R and note that E
should be divided by I. Thus 12 volts 3 amperes = 4 ohms resistance.
WORK AND POWER IN AN ELECTRIC CIRCUIT
a. A given amount of electricity can do a definite amount of work. There is a
simple relationship between the work produced and the voltage and amperage in a
Power is the rate at which work is done per unit of time.
(2) The watt (W) is the unit of electrical power. It is equivalent to one
ampere of current flowing at a pressure (EMF) of one volt.
(3) The power rule formula determines the power of continuous DC (the
amount of work the current can do per second):
power in watts = amperage times voltage
(4) The power of most electrical machinery is measured in terms of a unit
called a horsepower: 746 watts are equal to one horsepower.
b. The most common loss of power in electrical work is due to the heat
developed when current is flowing through a resistance. The greater the resistance of
the circuit, the greater is the rate of heat production. A combination of the power
formula and Ohm's law (E = I X R) gives the power loss due to heat production of a